The present invention generally relates to a diffusion barrier coating for a Si-based substrate. The present invention also relates to an apparatus comprising a Si-based substrate and a diffusion barrier coating on the Si-based substrate.
Silicon-based materials, such as silicon nitride (Si3N4), silicon carbide (SiC), molybdenum silicides, niobium silicides, and their composites have been developed for ultra-high temperature gas turbine engine hot section components, such as blades, nozzles, and combustors. Such materials have superior high temperature performance, as compared with Ni-based superalloys. However, in the gas turbine environment, such silicon-based materials are prone to excessive oxidation to form a silica layer, and the subsequent rapid loss of the silica layer. Excessive oxidation of the substrate results from the infiltration of substrate constituents, coating constituents, or environmental contaminants, into the silica layer. In particular, when cationic impurities having a valance <+4 are incorporated into the silica layer, the network of the silica glass is broken down, and the oxygen diffusion rate of the silica layer is greatly increased, thereby promoting further oxidation of the substrate. This in turns leads to increased loss of silica due to the reaction of the silica layer with water vapor in the environment to form volatile or gaseous SiO and Si(OH)x species. The combination of excessive oxidation of silicon-based components and loss of the resulting silica layer leads to recession of the components, reduced load-bearing capability, and shortened lifetime.
Attempts have been made to inhibit oxidation of silicon-based components by applying an environmental barrier coating to prevent direct exposure of the substrate to oxygen and water vapor in the gas turbine engine environment. However, a silica layer still inevitably forms at the interface between the substrate and the environmental barrier coating, and the reaction of the silica layer at the substrate-environmental barrier coating interface with constituents of the environmental barrier coating degrades the protective property of the silica layer. Similarly, the silica layer may be degraded by reaction with constituents of the substrate with which it is in contact. Further, silica layers of the prior art which are in direct contact with Si-based substrates continue to grow in thickness until through-thickness cracks develop, which may lead to spallation of an entire environmental barrier coating.
U.S. Pat. No. 6,071,622 to Beesabathina, et al. discloses a stabilized two-phase glass diffusion barrier for a superalloy substrate. The barrier comprises a base layer that may be formed from an alumina or borosilicate solgel, a two-phase glass diffusion barrier layer on the base layer, and a two-phase glass sealant layer on the diffusion barrier layer.
U.S. Pat. No. 6,312,763 B1 to Eaton, Jr., et al. discloses a silicon-based substrate with an environmental/thermal barrier layer comprising a bond layer of silica or silicon metal on the substrate, an intermediate layer on the bond layer, and a barrier layer on the intermediate layer. The intermediate layer comprises a material such as silica, mullite, an aluminosilicate, an yttrium silicate, or silicon metal; while the barrier layer comprises predominantly Y2O3, balance SiO2.
Possible disadvantages of the '763 patent include the following. Silicon metal undergoes softening at the temperatures experienced during service. Further, such a coating is susceptible to degradation following diffusion of substrate constituents into the coating. In particular, the silicon metal and silica of the bond layer, being in direct contact with the Si-based substrate, can react with constituents of the substrate at temperatures experienced during service to form a low-viscosity silicate glass that is not an effective barrier to diffusion of oxygen into the substrate, whereby the substrate is not protected from oxidative damage.
U.S. patent application Publication No. 2003/0003328 A1 discloses a coating for a non-metallic substrate, wherein the coating comprises a silica layer on the substrate, a layer of mullite (or mullite plus barium strontium aluminosilicate (BSAS)) on the silica layer, a layer of an alkaline earth aluminosilicate on the layer of mullite or mullite plus BSAS, and a thermal barrier coating comprising yttria stabilized zirconia on the layer of alkaline earth aluminosilicate. As described hereinabove, the silica layer of such a coating, being in direct contact with the substrate, is susceptible to degradation following diffusion of substrate constituents into the silica layer.
Thus, while various environmental barrier coating systems have been applied to Si-based nonmetallic components in an attempt to decrease oxidation of the substrate and to reduce the rate of recession, there remains the problem of excessive oxidation of Si-based nonmetallic components due to exposure of the silica layer to constituents of the environmental barrier coating and to constituents of the substrate during service conditions.
As can be seen, there is a need for a diffusion barrier coating that can protect a Si-based substrate from oxidative damage during gas turbine engine service conditions (e.g., thermal cycling to temperatures of 1200 to 1350° C.). There is an additional need for a diffusion barrier coating for a Si-based substrate, wherein the diffusion barrier coating includes a first isolation layer, disposed between the Si-based substrate and an oxygen barrier layer, to isolate the oxygen barrier layer from the Si-based substrate. There is also a need for a component comprising a diffusion barrier coating on a Si-based substrate, and an environmental barrier coating disposed on the diffusion barrier coating, wherein the diffusion barrier coating includes a second isolation layer disposed between the environmental barrier coating and the oxygen barrier layer to isolate the oxygen barrier layer from the environmental barrier coating.
The present invention provides such coatings and components, as will be described in enabling detail hereinbelow.